1. Field of the Invention
The present invention relates to an identification method for a generated position of a dynamic false contour for restraining dynamic false contours generated on a display using subfields such as a plasma display panel, a processing method for an image signal, and a processing apparatus for an image signal.
2. Description of the Related Art
As a conventional basic gradation display method, six subfields arranged in an order of luminance ratio of 1:2:4:8:16:32 are combined for representing one field with 64 gradations as described in FIG. 8 of Japanese Patent Laid-Open Publication No. Hei. 7-271325, for example. The order of turning on for sustain discharge is fixed for the six subfields, and the order is identical for the gradation levels with respect to the time axis.
However, when a motion picture is displayed with this method, there is such a problem that unevenness of light emission becomes remarkable in terms of time, and thus large disturbance occurs in the gradation when bit-carry gradation levels (such as 63 and 64, 31 and 32, and 15 and 16) exist. This disturbance in the gradation is called as a dynamic false contour.
As a result, it is necessary to identify a position where the dynamic false contour is generated, and then to conduct signal processing for restraining the dynamic false contour at the identified position to obtain proper image quality.
As a method for identifying a position where a dynamic false contour is generated, image data corresponding to one field period are stored in a frame buffer, then, image data in the next field period are compared with the image data in the stored field period, and consequently motion vectors are detected based on the comparison result.
Also, as a method for restraining the dynamic false contour, the following methods described in FIG. 16 to FIG. 23 in Japanese Patent Laid-Open Publication No. Hei. 7-271325 have been applied.    (a) A method for switching order of individual subfields    (b) A method for increasing the number of subfields constituting one field so as to secure two or more types of combinations of light-emitting subfields when a gradation is represented.
Further, a method for spatially diffusing the disturbance in the gradation by signal processing such as error diffusion method is applied.
In addition, there is a method called as equalization pulse method described in “Display and Imaging” 1997, Vol. 5, pp. 229–240 as an alternative method for restraining a dynamic false contour. In this equalization pulse method, an amount for emitting light is added to or subtracted from an original signal when a disturbance of gradation is expected to be observed if the line of sight moves on a display where subfields are used, and consequently large disturbance in gradation is restrained.
However, since the method for identifying a position where a dynamic false contour is generated by detecting motion vectors requires a frame buffer with a large capacity, there is such a problem that the cost remarkably increases. Also, it is practically impossible to detect a large number of motion vectors all at once, and simultaneously, it is difficult to detect a sudden and large change of motion vectors. As a result, a detection error occurs, and consequently there is such a problem that an image is extremely degraded as the result of processing for restraining dynamic false contour based on the detection error.
Also, in the conventional processing for restraining the dynamic false contour for multi-gradation display, there are the following problems. First, when there are N subfields, it is possible to represent 2N gradation levels. However, when the number of the constituting subfields is increased to more than N on a plasma display panel for restraining the dynamic false contour by reducing unevenness of light emission in terms of time, the sustain discharge period becomes short, and consequently the luminance decreases. As a result, it is impossible to increase the number of the subfields without reducing the luminance. Secondly, when the number of the divided subfields is increased, since disturbance in the gradation occurs for a specific gradation levels, it is impossible to prevent the disturbance in the gradation for the specific gradation levels.
Additionally, the signal processing for restraining the dynamic false contour by the conventional error diffusion method has the following problems. First, since the signal processing is applied to an input signal whether the dynamic false contour is generated or not, the input signal in a region where the dynamic false contour is not generated is degraded. Secondly, since there is no regularity in the disturbance in the gradation diffused by the error diffusion method, it is impossible to predict influence of the diffused disturbance in the gradation in advance.
Further, the processing for restraining the dynamic false contour by the equalization pulse method has the following problems. First, an input signal is corrected according to a motion speed of a figure for reducing disturbance of an image recognized by the eye by detecting the motion of the figure in the input signal. Since the precision of detecting the motion vectors decreases for some input image, a signal correction error may decrease the quality of the motion picture. Secondly, since it is assumed that the line of sight follows a moving figure, disturbance of the image due to the corrected signal may be recognized when the line of sight does not follow the figure.